1. Field of the Invention
This invention relates to a zoom lens system, and more particularly to a zoom lens system #or a home video camera or an electronic still camera.
2. Description of the Prior Art
In a video camera, the image taking size is being reduced to 1/4 inches from 1/3 inches, and an attempt simplify the structure of the zoom lens system and reduce the number of the lens elements in the zoom lens system has been made in order to miniaturize the zoom lens system for a video camera in response to reduction in the image taking size. For example, the majority of known zoom lens systems having an F-number of 1.6 to 1.8 and a zoom ratio of .times.8 to .times.10 consist of four lens groups, the first and third lens groups being fixed and the second and fourth lens groups being movable. In such zoom lens systems, the lens system is zoomed by moving the second lens group in the direction of the optical axis and change in the image-forming position generated by the movement of the second lens group is compensated for moving the fourth lens group in the direction of the optical axis.
However the four lens group zoom lens systems are disadvantageous in the following point. That is, when the zoom lens system is zoomed from the wide-side end the tele-side end or from the tele-side end to the wide-side end, the fourth lens group is fed forward (toward the object) and then fed rearward. Accordingly in a middle focal length range, the fourth lens group is positioned close to the third lens group and light bundles traveling toward peripheral portions of an image emanate from the fourth lens group at a relatively small distance from the optical axis of the fourth lens group. As a result, aberrations in the middle focal length range differ from those in the range near the wide-side end or the tele-side end and it is difficult to suppress fluctuation in aberrations over the entire focal length range.
When the refracting power of the fourth lens group is increased in order to shorten the focal length at the wide-side end and enlarge the angle of view, the distance of movement of the fourth lens group is shortened and the overall size of the lens system is reduced. However it increases fluctuation in aberrations generated by movement of the fourth lens group and makes it further difficult to compensate for aberrations over the entire focal length range.
In order to increase the zoom ratio, it is necessary to increase the distance of movement of the fourth lens group as well as that of the second lens group, and when the distance of movement of the fourth lens group is increased, the fluctuation in aberrations is increased and it becomes difficult to compensate for aberrations over the entire focal length range.
In U.S. Pat. No. 5,299,064, there is disclosed a five lens group zoom lens system in which aberrations are compensated for over the entire focal length range. The five lens group zoom lens system consists of a positive first lens group, a negative second lens group, a positive third lens group, a positive fourth lens group and a negative fifth lens group, the second and fourth lens groups being movable and the other lens groups being fixed. In the zoom lens system, aberrations are compensated for over the entire focal length range and the size of the overall lens system is reduced by limiting the focal length and the like of the second lens group within a predetermined range, and the zoom ratio and the F-number of the zoom lens system are about .times.8 to .times.10 and about 2.0, respectively. Further the zoom lens system is zoomed by moving the second and fourth lens groups and is focused by moving the fourth lens group.
However in either of the aforesaid four lens group and five lens group zoom lens systems, the angle of view is 58.degree. at most and the zoom ratio is .times.10 at most. There is an increasing demand for a zoom lens system having a larger angle of view and a larger zoom ratio.